Traditionally nuclear medicine focused on the generation of two-dimensional images constructed from a volume of interest, although a variety of imaging devices have been proposed to perform three-dimensional imaging using an Anger type gamma camera. Since about 1980, several nuclear camera manufacturers have commercially introduced rotational type nuclear camera systems featuring a rotatable detector or camera head having a parallel hole collimator for data collection and an associated digital computer. The computer processes the collected data and performs known CT-type algorithms for reconstructing tomograms, i.e., two-dimensional images of a patient along a plane intersecting the patient.
One such ECT system is described in U.S. Pat. No. 4,426,578 to Bradcovich, et al. and assigned to the assignee of the instant application. Bradcovich, et al. invented a system that features a counterbalanced C-arm supporting a camera head at one end thereof for rotation about a longitudinal axis through a patient. The radial distance between the camera head and the longitudinal axis is rendered adjustable by displacement of the C-arm along a circumferential path relative to a so-called carrier member which rotatably attaches the C-arm to a stationary base. Another ECT apparatus is described in U.S. Pat. No. 4,216,381 to Lange which features a rotatable detector head supported by a pair of elongated frame members that pivotally support the detector head as it rotates about a longitudinal axis through a patient. In the Lange arrangement, the radial distance between the longitudinal axis and the detector head is adjusted by tilting the elongated frame pair which are mounted within a circular frame supported, in turn, by a pair of upright stanchions.
Regardless of the type of apparatus used to support the rotatable camera head, the reconstruction algorithms are always based on the collection of projection data acquired at a set of viewing angles about the patient by the rotating detector and subsequent back-projection of the data by means of the computer. For a detailed discussion of the general approach see, for example, Keyes Jr., "Computed tomography in nuclear medicine". Accurate retracing of projection lines during back-projection is essential to assuring good image resolution and quality. In ECT operation a major degradation in image quality is caused by deviations between the actual photon paths of the data collected and their paths traced during back-projection.
Regardless of the specific type of reconstruction algorithm used to generate the desired planar images or tomograms, the techniques uniformly assume that the camera head always follows the expected path. In practice, however, the actual path of the detector deviates from the expected path so that its position at each angle will exhibit some offset due mostly to mechanical flex in the detector support system and, to a lesser degree, to electronic image plane shift. This is caused by slight variances among the camera's Photo Multiplier Tubes' operation which originates from the varying orientation between the detector plane and the earth/ambient magnetic field. These deviations have largely been ignored resulting in errors in the reconstructed image.
Errors of this sort are generally unavoidable. It has been found, however, that they are nonetheless predictable since the amount of deviation in any particular system is measurable. While the amount of deviation varies as a function of viewing angle the errors tend to be relatively constant from rotation to rotation over a long period of time.